Cytochromes are heme proteins essential for the aerobic and anaerobic growth of most organisms, including humans and human pathogens. The assembly of c-type cytochromes occurs by one of three pathways, called systems I, II, and III. Defects in these pathways are the basis for certain human genetic diseases (in system III) and because only prokaryotes, plants, and protozoa use systems I or II, these pathways are ideal extracytoplasmic targets for antimicrobial agents (against pathogens). Many organisms that can not be cultured and/or genetically manipulated use these pathways. A specific inhibitor molecule would facilitate an understanding of how they grow and survive. Since cytochrome c is directly involved in apoptosis, and implicated (by mitochondrial bioenergetic functions) in aging and neurological manifestations, inhibitors of cytochrome c synthesis would provide valuable tools to study these processes. A major breakthrough in the field has come from the PIs lab in engineering the complete pathways of systems I and II to function in Escherichia coli. Moreover, the PIs lab has developed recombinant, inducible cytochrome c reporters, as well as very sensitive chemiluminescent (ECL) methods that detect subpicomolar levels of the cytochrome c product. Here it is proposed to advance these engineered E.coli for ultimately the discovery of specific small molecule inhibitors of the pathways. Modifications of the strains and the culture conditions will optimize and validate 96 and 384 well microtiter plate technologies for cytochrome c product detection. Two different assays will be optimized: i) the ECL-based high-throughput detection of cytochrome c in whole recombinant E.coli cells and ii) the immunological detection of holo cytochrome c (his-tagged and as a PhoA fusion) with antisera already generated. These screens will then be used for the discovery of small molecule inhibitors towards one or all of the three pathways. Strains and methods already developed will establish inhibitory specificity towards the pathways and will dissect the step(s) inhibited in the pathways. The PIs group has recently shown that large molecule, heme analogs inhibit specific steps, but these analogs require specific porins for access. The cell-based assays will also be validated by detection of this inhibition and by the use of known heme biosynthesis inhibitors. Thus, the HTS will also discover new inhibitors of heme biosynthesis, and the PIs group has developed the tools to quickly determine which enzyme (of nine) is inhibited in heme synthesis.